Epoxides are highly reactive chemical compounds which, as a result of their reactivity, can be used in a wide variety of applications. Unfortunately, due to the reactivity of epoxides, they are often difficult to prepare with high selectivity and in high yields. Ethylene is the only olefin which has been successfully oxidized employing molecular oxygen on a commercial scale to produce an epoxide.
Preferred catalysts employed for the oxidation of ethylene to produce ethylene oxide comprise silver on solid supports. When such catalysts are employed for the oxidation of other olefins such as styrene, epoxides are obtained, if at all, only in low yields and with relatively low selectivity. In addition, significant quantities of various higher oxidation products (up to and including carbon dioxide and water) are obtained.
For example, U.S. Pat. No. 2,992,238 (assigned to the Dow Chemical Company, issued Jul. 11, 1961) indicates that "all attempts to find a catalyst that would allow direct oxidation of higher olefins to epoxide in the same manner that silver catalyst works for ethylene led to failure . . . " (col. 1, lines 22-25). There is then disclosed a particular type of supported silver catalyst useful for oxidation of styrene, which catalyst is prepared by a reduction process using a polyhydric alcohol compound as both a reductant and as an agent for promoting adhesion between molecular silver and the support on which it is deposited. Catalyst preparation, therefore, requires special attention and involves the use of additional chemicals such as diethylene glycol.
Conversions in the range of 3 up to only 13.4 percent are reported for these polyhydric alcohol-prepared catalysts. Selectivities to styrene oxide reported are as low as 47% when higher conversions are achieved, and no higher than 85% at the very low conversion levels. Thus, mediocre performance is obtained employing catalyst which requires special considerations when being prepared.
Japanese Kokai Patent No. Sho 48[1973]-40739 reports an improvement over the disclosure of '238 by using a catalyst obtained by adding large quantities of barium peroxide to silver oxide. Ratios of barium to silver fall in the range of about 0.01:1 up to 1:1. Indeed, the authors indicate that a barium peroxide-free silver catalyst exhibits hardly any styrene oxide-producing activity in a process whereby styrene is oxidized.
While the authors of the above noted Japanese reference indicate that barium peroxide-containing catalyst can produce styrene oxide in an extremely high yield, the examples reveal styrene conversions in the range of about 14 up to 50 mol %, with selectivities in the range of only about 60 mol % at the higher conversion levels, with somewhat higher selectivity, up to 86 mol % at lower conversion levels.
The Japanese authors of the Japanese Kokai reference discussed above have also published a detailed scientific report of their work on the vapor-phase oxidation of styrene to styrene oxide in Nippon Kagaku Kaishi, 1977(11), pp. 1603-1609. This study investigates the affect of catalyst additives such as calcium nitrate, sodium hydroxide, magnesium powder, barium peroxide, tin hydroxide, diphosphorus pentoxide and potassium hydroxide on styrene conversion and selectivity to styrene oxide. The styrene conversions reported fall in the range of 0.1 up to 17% with selectivities to styrene oxide of 50 mol % at high styrene conversion, and up to 78% at the lower conversion levels, with the best performance reported employing the barium peroxide additive.
Alternate routes to styrene oxide and oxides of styrene derivatives include the non-catalytic oxidation of styrene with peroxides. Such processes are not only uneconomical, but are also hazardous due to the large quantities of peroxide required for desirable conversion levels.
It would, therefore, be desirable to be able to catalytically oxidize styrene or styrene derivatives to produce the corresponding oxide directly in high yields and with high selectivity. Such processes would provide large quantities of highly reactive olefin derivatives which would find a wide range of uses, such as for example, as polymer cross-linking agents, as reactive chemical intermediates, as precursors for the production of organic solvents, and the like.